1. Field of the Invention
The present invention relates to a signal processing circuit for processing a television signal, for example, and more particularly to an APC filter for a killer detector contained in an IC chip.
2. Description of the Related Art
A detector circuit of a TV signal processing circuit, by convention, is realized by a circuit arrangement as shown in FIG. 2. In the circuit arrangement, a transmission conductance of a demodulator consisting of transistors 1 to 6, and current sources 7 and 8, and a capacitance of a capacitor 9, cooperate to determine a time constant of a filter circuit. In a conventional detector circuit based on the above circuit arrangement, to increase a time constant, a smoothing capacitor of a large capacitance is externally coupled with the chip. In another conventional detector circuit of the type in which the capacitor 9 is contained in the IC chip, an output current of the current source 8, which feeds an operating current of the demodulator, is made small by reducing an output current of the current source 8, which feeds an operating current of the demodulator.
Thus, in the conventional IC circuit, to increase a time constant of the filter circuit, the large capacitance capacitor 9 must be externally coupled with the IC chip. The reason for this is that the capacitor of a large capacitance cannot be fabricated into the IC chip. Therefore, the number of externally coupled components is increased.
In the approach in which the capacitor 9 of a small capacitance is contained in the IC chip, to obtain a desired filter time constant, an output of the current source 8 must be set to an extremely small value. This results in a degradation of a frequency characteristic of the demodulator.